That's the low pressure cutoff switch, meaning the switch will close and power the compressor clutch once the minimum system pressure has been reached (seems to be ~ 5-9 psi). However, the switch you have is not OEM original; somebody found a replacement, likely because the original switches fail over the decades and no longer make consistent contact when pressurized.
I've tried pretty hard to find a replacement for the OEM one in the USA, but have not found a source. The thread pitch is different than most other switches with the same thread diameter. Thus, I keep the switch wires permanently jumpered with a piggyback spade connector.
The standard Montreal wiring diagram does not address the A/C system. The one that claims to cover the A/C is not 100% accurate. The official early Alfetta wiring diagram (with A/C) gives the best representation for the Montreal's A/C system, as the two are very similar.
Finally, a good belt size is a 10mm wide x 975 mm long belt (x2), which will put the compressor just past the midway point of its slot free travel.